Radiation dosimeter

ABSTRACT

A dosimeter for measuring high energy ionizing radiation utilizes a metal-oxide-semiconductor (MOS) structure as the sensing element. Advantageously, the MOS structure comprises a silicon wafer a portion of one surface of which is oxidized and then provided with an aluminum electrode. In one embodiment, provision for heating the MOS structure and for measuring its temperature is incorporated in the MOS structure. After exposure to ionizing radiation the MOS structure is heated with an ammeter connected between the silicon wafer and aluminum electrode. The integrated current flow is then a measure of the radiation dose received by the structure.

I United States Patent l 13,569,704

[72] Inventor John Mitchell 3,225,198 12/1965 Mayer 250/833 Sumnut, N-J.3,396,318 8/1968 Chow 250/83.3X [21] p 752940 Primary ExaminerRa1ph G.Nilson [22] F11ed Aug. 15, 1968 Asszstam Examiner-Davis L1 Willis [45]Patented Mar. 9, 1971 A R l dA A d [73] Assignee Granted to I LS ktomicEnergy Commission omey 0 an n arson under the provisions of 42 U.S.C.2182 [54] RADIATION DOSIMETER ABSTRACTtA dosimeter for measuring highenergy ionizing 5 Claims, 4 Drawing Figs. radiation utilizes ametal-oxide-semlconductor (MOS) structure as the sensing element.Advantageously, the MOS struc- [52] U.S.Cl. 250/833, tux-e comprises aSilicon wafer a portion f one surface of 250/83 317/234 which isoxidized and then provided with an aluminum elec- [g] G01t1/02 trod: Inone embodiment, provision f heating the MOS e 0 Sean 250/833, structureand f measuring its temperature is incorporated n 71-5, 317/235/27'250/83 the MOS structure. After exposure to ionizing radiation the MOSstructure is heated with an ammeter connected between [56] Referencescued the silicon wafer and aluminum electrode. The integrated cur-UNITED STATES PATENTS rent flow is then a measure of the radiation dosereceived by 3,117,229 1/1964 Friedland 250/833 the structure.

RADIATION DOSMETER GOVERNMENT CONTRACT The invention herein claimed wasmade in the course of, or under contract with the Air Force.

This invention relates to dosimetry and relates more particularly to themeasurement of high-energy, ionizing radiation, such as X-rays and gammarays by use as the sensing element of a structure including asemiconductive crystal, a portion of one surface of which includes inturn an insulating layer, and an electrode to form ametal-insulator-semiconductor structure.

BACKGROUNE OF THE INVENTION Detection and measurement of high energyradiation have long been important in the physical and the lifesciences, and a wide variety of techniques have been evolved over theyears.

. One of the most important in current use involves thermoluminescence.

As currently practiced, thermoluminescence dosimetry involves theirradiation of a sensing element, such as manganese-activated calciumfluoride or doped lithium fluoride, with the radiation to be measuredand thereafter heating the irradiated element to induce luminescencewhich is supplied to a photomultiplier which provides a current whichwhen integrated is a measure of the radiation dose.

While sensitive, this technique is complex and requires expensiveequipment to instrument.

SUMMARY OF THE INVENTION In accordance with the present invention, thesensing element used comprises a sandwich of a semiconductive crystal,an insulating layer, and a suitable metal electrode. Advantageously, thesandwich comprises a silicon crystal, an intermediate layer of silicondioxide, and an aluminum or gold on chromium electrode. When the sensingelement is exposed, the ionizing radiation serves to createhole-electron pairs within the insulating layer. Some hole-electronpairs recombine while others are trapped in electron and hole traps.Because the electrons are considerably more mobile than the holes moreelectrons than holes may escape from the silicon dioxide layer leaving apositive space-change in the silicon dioxide. This charge pattern isquite stable with time so long as the device is-kept at or near roomtemperature. However, when the device is heated, for example, to 200(1., there will flow in an ammeter, connected between the metalelectrode and the semiconductive crystal, current which when integratedis a measure of the radiation dose received by the MOS structure.

Advantageously, there is included in the silicon crystal a diffusedresistive portion which, when supplied with current, acts as a heater toheat the silicon chip. Optimally, the silicon crystal also includes inone portion a diffused PN junction a measure of whose impedance canbeused to determine the temperature of the silicon crystal andto'control the heater power to give a desired rate of increase oftemperature during the heating cycle.

DESCRIPTION OF THE DRAWING DETAILED DESCRIPTION OF THE INVENTION Withreference now to FIGS. 1 and 3, a silicon crystal ii is provided with acentral portion where one surface is oxidized to provide an oxide layer12 over which lies the field electrode 13. An electrode M makes lowresistance ohmic connection to the opposite surface of the crystal.

Typically, the crystal ill may be a wafer mils square and 20 mils thick,the bulk of which is of N-type with a relatively high resistivity of 4ohm centirneter. The oxide layer 12 may be 60 mils square and 2 micronsthick. The electrodes 13 and 14 may be of aluminum or gold on chromium.

Additionally, along three sides of the oxide layer there extends withinthe crystal the P-type resistivity region l5, typically formed by theselective diffusions of acceptor impurities to produce the desiredresistivity. Electrodes 16 and 17 make low resistance connection toopposite ends of the region 15.

The region 15 together with its associated electrodes 16 and 17 servesas a heater such that by the application of a voltage between electrodes16 and 17 a current flows in region 15 adequate to heat the centralportion of the crystal over which lies the oxide layer to a temperatureof at least 200 C. in a few seconds, the faster the better usually.

Along the fourth side of the oxide layer, the crystal includes a P-typezone 18 which forms with the N-type bulk aPN junction 119, as seen inFIG. 2. A low resistance ohmic connection 20 is provided to the zone 18and the current flow across the PN junction 19 in response to either aforward or reverse bias can be used to monitor the temperature of thecrystal.

In FIG. 4, there is shown how the device of FIG. 1 would beinterconnected to measure the amount of ionizing radiation to which thedevice has been exposed. The device is represented by its equivalentcircuit in which the suffix A is added to the reference numerals used inFIG. I to denote the corresponding elements in the equivalent circuit.

Specifically the heater circuit is completed by connecting a suitablevariable DC voltage source 21 between electrodes 16 and 17 to providecurrent through the resistors 15A.

The detection circuit is completed by connecting between electrodes 13and M a suitable ammeter 22 capable of measuring and integratingcurrents of the order of picoamperes.

Finally, the thermometer circuit is completed by connecting a DC voltagesource 23 poled to bias the PN junction WA in reverse and an ammeter 24between electrodes 14 and 20. If desired, there may be includedapparatus which uses the current flowing between electrodes 14 and 20 tocontrol the voltage of the heater supply 21 and, in turn, thetemperature of the crystal.

When a metal-oxide semiconductor structure is exposed to ionizingradiation, hole-electron pairs are generated substantially uniformlythroughout the oxide layer. Some of the holeelectrons pairs willrecombine while others will exist in the oxide layer as trapped holesand electrons in the form of com pensated charge. (A positivespace-charge also occurs in the oxide and arises because the electronsgenerated by the radiation are much more mobile than the correspondingholes and tend to drift or diffuse out of the oxide more readily. Thispositive space-charge is not, however, directly involved in theoperation of the dosimeter.)

It has been found experimentally that the compensated trapped charge ismuch larger than the positive space-charge and depends on the radiationdose but is virtually independent of the voltage applied between thesilicon and metal electrode during irradiation.

It has been found that when heated the irradiated device will produce athermally stimulated current in an ammeter connected between the metalelectrode and the silicon body. The direction of this current is such asto require a net injection of electrons into the oxide layer from themetal electrode.

The observed effects are consistent with the following proposedmechanism although a complete understanding of the exact mechanisminvolved is not necessary for the practice of the invention, and sol donot intend to be bound to any particular mechanism.

-When the structure is heated, the electrons in the compensated chargewhich are trapped about 1 electron volt below the oxide conduction bandare released. Subsequently, some electrons diffuse out of the oxide butmost recombine with more deeply trapped holes. The recombination processis a luminescent one. The light is absorbed by both the metal electrode13 and the silicon electrode 11 producing photoinjection of electronsinto the oxide. The yield from the two electrodes is not the same,however, because of asymmetry in the properties of the metal-oxide andsilicon-oxide interfaces. The metal electrode, becaUse of the smallerbarrier height at its interface (about 3.2 electron volts) as comparedto the barrier height of the silicon interface (about 4.5 electronvolts) and/or more favorable absorption characteristics, injects a muchlarger number of electrons into the oxide than does the semiconductor,producing a net current flow as observed.

The proposed explanation suggests that it is advantageous for largecurrent flow to choose the various materials involved so that thebarrier height of the metal electrode-insulator interface is lowrelative to that of the semiconductor-insulator interface.

The integrated current flow through the structure is found to depend onthe radiation dose received, increasing approximately linearly withdosage for doses up to about 1 megarad. From the experiments performed,it is estimated that approximately 10- cm of silicon oxide will producean integrated current of 4 X lcoulombs after an exposure of about 3.5 X10 rads. This amount of charge is large enough to be measuredquitereadily with available amrneters and, hence, the integratedthermally stimulated current can be used as a measure of the radiationdose absorbed by the structure, once the particular structure has beencalibrated by irradiation with known dosages.

The most appropriate package for the sensing element depends on the typeof radiation which it is designed to measure. The packaging, typically,is designed to minimize interception of the radiation to be measuredand, in some instances, measures may be taken to intercept otherpossible ionizing radiation. For energetic radiation greater thanapproximately 0.5 MeV, a standard transistor package can be employedwhile for relatively low energy radiation, simple beam lead packaging,including a protective layer of silicon-oxide and silicon-nitride andbeam leads comprising layers of titanium, platinum and gold could beused.

Moreover, for passivating purposes it may be advantageous to provide anoxide layer over all of the top surface of the silicon wafer and to formthe various desired P-type zones by diffusing through openings forrnedin the oxide layer in the manner known to workers in the art.

It should be evident that the specific embodiment described is merelyillustrativepf the general principles of die invention and that variousmodifications may be made without departing from the spirit and scope ofthe invention. in particular, it is feasible to employ in themetal-insulator semiconductor structure various other combinations ofmaterials and with other geometrical arrangements of the MOS structure,heater and thermometer. It should also be feasible to adapt the involvedphenomenon outside the field of dosimetry in any application where it isdesirable to store the effect of high energy radiation and to providesubsequently a current which is a measure of such high energy radiation.Furthermore, the stability'of the storage may be enhancedby maintainingthe device at temperatures below room temperature.

I claim:

1. A dosimeter for measuring ionizing radiation comprising:-

a semiconductive device comprising a semiconductive crystal, aninsulating layer over a portion of'one face of said crystal and a metallayer over said insulating layer and a low resistance connection to saidcrystal, the device being adopted to be irradiated with the radiation tobe measured for creating hole-electron pairs in the insulating layer;

means for heating the semiconductive device after irradiation; and

means connected between the metal layer and the low-resistanceconnection of said device for measuring the current flow therebetween inresponse to the heating of the semiconductive device.

2. A dosimeter in accordance with claim 1 in which the semiconductivecrystal is of silicon and the insulating layer is of silicon-oxide.

3. A dosimeter in accordance with claim 1 in which the heating meanscomprises a separate region of the crystal and means for the passage ofcurrent through such region for heating the crystal.

4. A dosimeter in accordance with claim 3 in which the crystal alsoincludes a PN junction and which includes means for measuring theimpedance of said junction to provide an indication of the temperatureof the crystal.

5. The method of measuring radiation comprising the steps of:

exposing to the radiation to be measured a semiconductive devicecomprising a semiconductive crystal with an insulating layer on onesurface and a metal electrode over said layer;

thereafter heating the exposed semiconductive device to a temperature inexcess of 200 C.; and

while heating the device measuring the current flow between the metalelectrode and the semiconductive crystal.

2. A dosimeter in accordance with claim 1 in which the semiconductivecrystal is of silicon and the insulating layer is of silicon-oxide.
 3. Adosimeter in accordance with claim 1 in which the heating meanscomprises a separate region of the crystal and means for the passage ofcurrent through such region for heating the crystal.
 4. A dosimeter inaccordance with claim 3 in which the crystal also includes a PN junctionand which includes means for measuring the impedance of said junction toprovide an indication of the temperature of the crystal.
 5. The methodof measuring radiation comprising the steps of: exposing to theradiation to be measured a semiconductive device comprising asemiconductive crystal with an insulating layer on one surface and ametal electrode over said layer; thereafter heating the exposedsemiconductive device to a temperature in excess of 200* C.; and whileheating the device measuring the current flow between the metalelectrode and the semiconductive crystal.